The invention relates to a heat-wire type airflow quantity measuring apparatus, which is used as one means for detecting the operating state of an automobile engine and which comprises intake airflow quantity detecting means such that even if the voltage is not stable, for example, accurate measurement data can be supplied to the engine control, which comprises a microcomputer, for accurate engine control and accurate control of the fuel injection amount and the ignition timing, etc. that is most suitable for the engine operating state.
The engine operating conditions such as the fuel injection amount and the ignition timing, etc. are calculated by an engine control unit (ECU), which is comprised of a microcomputer, and, based on the calculation results, the injection amount and ignition timing are controlled. This kind of electronic control unit calculates the engine operation conditions based on detection signals from a plurality of sensors that monitor the engine. Examples of the kind of sensors that may be used are: a rotation sensor which outputs detection signals at specified angles of crankshaft rotation, an engine coolant temperature sensor, throttle opening sensor, an air/fuel ratio sensor which detects the oxygen density in the exhaust gas, and a battery voltage sensor. An example of a sensor that directly detects the engine operating state is an intake airflow quantity sensor. The engine control unit calculates the basic fuel injection amount and the ignition timing corresponding to the detected intake airflow quantity. These basic values are corrected based on the other detection data such as engine RPM and coolant temperature, and instructions are supplied to the fuel injection mechanism and the ignition timing control device.
An example of this kind of intake airflow quantity measuring means that is used in the engine control apparatus is the heat-wire type intake airflow measuring means disclosed in U.S. Pat. No. 3,975,951.
With this apparatus a heating current is supplied to a heater whose electrical resistance varies with changes in temperature and which is located in the air intake pipe. By measuring the changes in resistance, which is indicative of the changes in temperature resulting from the flow of air past the heater, it is possible to determine the quantity of air flowing in the intake air pipe. When the airflow is large, the rise in temperature of the heater is slower for a given heating current than when the airflow is small, therefore, it is possible to calculate the intake air quantity based on this heating current because in order to maintain the same temperature a heating current must increase in proportion to the quantity of the airflow.
With this kind of measuring device, however, the measurement signals are analog and in order to be used by the control unit they must first be converted to digital by an A/D converter. For this kind of control unit an extremely accurate A/D converter is required, which complicates the measurement signal processing means.